For anyone with even the slightest phobia about snakes, it might be best to look away now.

Fossil hunters working in an open-pit coal mine in Colombia have unearthed the remains of several giant prehistoric snakes, thought to be the largest ever to have slithered on earth.

Palaeontologist Dr Jonathan Bloch from the University of Florida, who discovered the fossil snake on an expedition to Colombia Link to this audio

The boa constrictor-like beasts, aptly named Titanoboas, weighed more than one and a quarter tonnes and measured at least 13m long from nose to tip. At their widest, the snakes would have come up to the waist of an adult human.

The partial skeletons of eight individuals were uncovered at the site, alongside the fossilised remains of what may have once have been the creatures' dinner: a 2m-long giant turtle and an ancient ancestor of the modern crocodile.

The skeletons were discovered when Carlos Jaramillo, a paleobotanist at the Smithsonian Tropical Research Institute in Panama, was invited to Cerrojon in northern Colombia, home to one of the world's largest open-pit mines, to date the rock formations there.

The fossils were encased in rock dating back 60m years, and so give scientists an unprecedented insight into the large animals that ruled the tropics after the sudden demise of the dinosaurs 65m years ago.

"Now we have a window into the time just after the dinosaurs went extinct and can actually see what the animals replacing them were like," said Jason Head, a paleontologist at the University of Toronto.

The fossilised remains were first flown to the University of Florida for analysis by Jonathon Bloch, an expert in prehistoric vertebrates, but he later got in touch with Professor Head, who specialises in ancient snakes.

During a video conference between the two, Bloch held up a vertebra from the snake. "I just about screamed. It was about as wide as a man's hand. The vertebra of a 17ft-long anaconda is only slightly more than an inch wide," Head told the Guardian.

The scientists, who will report the finding in the journal Nature tomorrow, were able to estimate the size of the snakes from the spinal vertebrae, and soon realised they were the largest ever seen.

The region of Colombia where the snakes were found was very different 60m years ago. It would have been a thick rainforest cut through by a network of rivers, resembling the modern Amazon.

The discovery has lifted a veil on the climatic conditions at the time the beasts were alive, since cold-blooded animals grow much larger in warmer environments. The largest cold-blooded animals alive today live in the tropics where it is hottest, but farther away from the equator they get steadily smaller.

Based on the snakes' size, the researchers calculated that the tropics were on average 5C warmer than they are today. The study marks a first in using the size of animals from their fossilised remains to infer the climate of the world tens of millions of years ago.

"Tropical ecosystems of South America were surprisingly different 60m years ago. It was a rainforest, like today, but it was even hotter and the cold-blooded reptiles were all substantially larger. The result was, among other things, the largest snakes the world has ever seen, and hopefully ever will," said Bloch.

The largest living snakes today are the giant anaconda and reticulated python, though these only rarely grow to 9m long.

Harry W. Greene, a snake expert at Cornell University in New York and author of Snakes: The Evolution of Mystery in Nature, said the discovery of the "colossal" Colombian snake had important implications for understanding life in the ancient tropics. "For decades herpetologists have argued about just how big snakes can get, with debatable estimates of the max somewhat less than 40 feet," he said.

David Gower, a zoologist at the Natural History Museum in London, said: "The fossil is much larger than the largest living snakes we find today such as the anaconda and pythons, which generally reach up to 6m, but exceptionally reach 7m for the anaconda and up to 9m for pythons. They were found alongside fossils of freshwater turtles, crocodiles and fish, suggesting that the titanic boa may have lived a similar lifestyle to modern anacondas."

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.